1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to WDM Mux/DeMux on cable and methods of making the same.
2. The Background of Related Art
Broadband internet has experienced a compounded annual bandwidth growth rate exceeding 30% over the last decade. The momentum shows no sign of slowing down as wireless broadband, due to the smart phones and portable devices, joins the game. Fibers that facilitate much of such land-line bandwidth supports in the past now challenge device and equipment development to keep up the speed to fill their available bandwidth capacities. In core networks, transport equipment can now support 40 Gbps and 100 Gbps per wavelength using dense wavelength division multiplexing (DWDM) carriers. 400 Gbps and even 1 Tbps per wavelength channel are being discussed and laboratory tried.
In order to meet the requirement of fast speed optical communication, multi-channels of difference wavelength is widely applied. Multiplexers/De-multiplexers (Mux/DeMux) is one of the components that are often needed in optical modules, such as C form-factor pluggable (CFP) as a key component to combine/split 4 signals, each operating at a different wavelength, into/from a common optical fiber. FIG. 1 a typical four channels of de-multiplexers (Demux) for CFP application. In the device, a light beam at multiple wavelengths traveling from a common port is separated into individual beams, respectively directed to individual channels. At each of the multiple channel, a dielectric thin film filter (TFF) transmits a selected wavelength band of the multiple-wavelength collimated light passed by a corresponding channel port from λ1 to λn and reflect all other wavelengths.
FIG. 2 shows a typical optical transmission and reflection spectra of a single-channel band-pass filter that transmits a wavelength λ and reflects wavelength from λ1 to λi−1 and λi+1 to λn. The reflected wavelength signals continue propagating to next channel ports, where the in-band signal is transmitted through the filter and the other signals are reflected and propagate along the path controlled by the reflectors and the filters. After multiple bounces in a zig-zag fashion, multiple channels are separated. The device can also be used as a multiplexer (Mux) with a common port to output a multiplexed light beam including all wavelengths added from the individual channels. This design has a number of advantages including low back-reflection, low loss and high reliability.
In FIG. 1, not all regime of the filter can be used for the Demux. The effective optical path is described in FIG. 3. When a light beam enters the filter 300, the direction of light propagation is shown as dotted line in FIG. 3. The maximum beam diameter that can transmit through as configured in FIG. 1 is Reff, which is determined by the refraction index of the medium and the filter. From the perspective view of the cross section of the filter, the regimes top above the top dotted line and beneath the bottom dotted line in the filter as shown in FIG. 3 are not effectively used for optical Mux/Demux application. As the evolution of high speed optical telecommunication and data communication, the dimension of optical transceiver decreases dramatically. To meet this requirement, the size of filters and pitches between two adjacent channels in WDM is reduced. As the pitch deceases, one problem emerges: how to align the filters in a ways that they would not contact each other.